1. Field of the Invention
The invention relates generally to a method of making molded composite articles from cellulosic materials and, more particularly, the invention relates to a method of making a consolidated cellulosic article exhibiting excellent surface hardness and surface smoothness characteristics.
2. Brief Description of Related Technology
One type of molded composite article is a wood composite which includes man-made boards of bonded wood sheets and/or lignocellulosic materials, commonly referred to in the art by the following exemplary terms: fiberboards such as hardboard, medium density fiberboard, and softboard; chipboards such as particleboard, waferboard; and oriented strandboard. Wood composites also include man-made boards comprising combinations of these materials.
Many different methods of manufacturing these wood composites are known in the art such as, for example, those described in Hsu et al. U.S. Pat. No. 4,514,532 and Newman et al. U.S. Pat. No. 4,828,643, the disclosures of which are hereby incorporated herein by reference. The principal processes for the manufacture of fiberboard include: (a) wet felted/wet pressed or xe2x80x9cwetxe2x80x9d processes; (b) dry felted/dry pressed or xe2x80x9cdryxe2x80x9d processes; and, (c) wet felted/dry pressed or xe2x80x9cwet-dryxe2x80x9d processes. Synthetic resins, such as amino resins, phenol-formaldehyde (xe2x80x9cPFxe2x80x9d ) resins, or modified PF resins, are often used as binders in these processes. Suitable phenolic resins may include those described in Teodorczyk U.S. Pat. No. 5,367,040, which is hereby incorporated herein by reference.
Generally, in a wet process, cellulosic fibers (e.g., woody chip material subjected to fiberization in a pressurized refiner to form wood fibers) are blended in a vessel with large amounts of water to form a slurry. The slurry preferably has sufficient water content to suspend a majority of the wood fibers and preferably has a water content of at least 95 percent by weight (wt. %). The water is used to distribute a synthetic resin binder, such as a phenol-formaldehyde resin over the wood fibers. This mixture is deposited onto a water-pervious support member, such as a fine screen or a Fourdrinier wire, and pre-compressed, whereby much of the water is removed to leave a wet mat of cellulosic material having, for example, a moisture content of at least about 50 wt. % based on the weight of dry cellulosic material. The wet mat is transferred to a press and consolidated under heat and pressure to form the molded wood composite.
A wet-dry forming process can also be used to produce wood composites. Preferably, a wet-dry process begins by blending cellulosic material (e.g., wood fiber) in a vessel with a large amount of water. This slurry is then blended with a resin binder. The blend is then deposited onto a water-pervious support member, where a large percentage (e.g., 50 wt. % or more) of the water is removed, thereby leaving a wet mat of cellulosic material having a water content of about 40 wt. % to about 60 wt. %, for example. This wet mat is then transferred to a zone where much of the remaining water is removed by evaporation by heat to form a dried mat. The dried mat preferably has a moisture content of about 10 wt. % or less. The dried mat is then transferred to a press and consolidated under heat and pressure to form a wood composite which may be a flat board or a molded product, for example. The product can be molded into various shapes or geometries depending on the intended use.
In a dry forming process, filler material, such as cellulosic fibers, is generally conveyed in a gaseous stream or by mechanical means. For example, the fibers supplied from a fiberizing apparatus (e.g., a pressurized refiner) may be coated with a thermosetting synthetic resin, such as a phenol-formaldehyde resin, in a blowline blending procedure, wherein the resin is blended with the fiber with the aid of air turbulence. Thereafter, the resin-coated fibers from the blowline can be randomly formed into a mat by air blowing the fibers onto a support member. Optionally, the fibers, either before or after formation of the mat, can be subjected to pre-press drying, for example in a tube-like dryer. The formed mat, typically having a moisture content of less than about 10 wt. % and preferably about 5 wt. % to about 10 wt. %, is then pressed under heat and pressure to cure the thermosetting resin and to compress the mat into an integral consolidated structure.
Steam injection pressing is a consolidation step that can be used, for example, under certain circumstances in dry and wet-dry process production of consolidated cellulosic composites. In steam injection pressing, steam is injected through perforated heating press platens, into, through, and then out of a mat that includes the synthetic resin and the filler material. The steam condenses on surfaces of the filler and heats the mat. The heat transferred by the steam to the mat as well as the heat transferred from the press platens to the mat cause the resin to cure. When compared with conventional pressing operations, steam injection pressing may, under certain circumstances, provide a variety of advantages, such as, for example, shorter press time, a more rapid and satisfactory cure of thicker panels, and products having more uniform densities.
Consolidation of mats containing conventional phenolic resins, however, often results in a final composite product exhibiting undesirable characteristics, such as poor bond formation and/or starved glue lines. In order to ensure good bond formation, it is desirable to have a uniform dispersion of the phenolic resin throughout the mat. However, because phenolic resins are water soluble, when mats containing such resins are consolidated (and especially when steam pressed) water present in the mat (or injected by steam injection means) solubilizes the resin. The solubilized resin undesirably migrates to regions of the mat such that the resin is no longer uniformly dispersed, resulting in a product having resin-starved regions and poor bond formation.
Poor bond formation also is attributable to the known phenomena of pre-cure (i.e., where the resin cures before the mat has hardened to an integral consolidated structure) and moisture retardation (i.e., where water present in the core or interior of the mat prevents the mat temperature from exceeding the evaporation temperature of water, 100xc2x0 C., thereby retarding the resin cure). Starved glue lines caused by excess penetration of the resin can oftentimes occur near the surface of the formed product where the resin has undesirably been washed from the surface of the mat and has migrated to edges of the mat or to the mat core. The absence of resin near the product""s surfacexe2x80x94hence, the onset of pre-cure and the presence of starved glue linesxe2x80x94results in a board product having soft surfaces, characterized by loose fibers and/or flaking of the cellulosic material that has not properly adhered to the body or core of the board.
Soft surfaces are particularly undesirable because it is difficult to paint soft surfaces or apply other finishing components (e.g., water resistant sealers) to soft surfaces. Furthermore, the integrity, surface smoothness, and aesthetic characteristics of the board product or molded product, either grained or smooth, are compromised by the soft surfaces.
Some prior attempts at preventing the occurrence of soft surfaces have included the application of a sealer, such as an acrylic sealer, to one surface of the fibrous mat prior to consolidating the mat in a heated press. Practically, however, these attempts do not address or teach one skilled in the art how to prevent the occurrence of soft surfaces on those surfaces that the sealer is not (or cannot be) applied to. Additionally, acrylic sealers may undesirably damage the surfaces of a molding die or press platens of a pressing apparatus. Other prior attempts at preventing the occurrence of soft surfaces have included the application of a material, such as drying oil, to both top and bottom surfaces of a consolidated product in an effort to xe2x80x9crepairxe2x80x9d the surfaces of the consolidated product and to increase the surface integrity of the product in subsequent post-pressing operation. Still other methods include blending a resin with a dried fibrous material.
All of the prior approaches to combating the occurrence of soft surfaces, however, are somewhat inefficient, inconvenient, time-consuming, and require considerable amounts of extra equipment and, therefore, add substantially to the cost of the finished product. Furthermore, some of the prior approaches are simply not suitable for continuous production processes. Accordingly, it would be desirable to overcome one or more of these problems. More specifically, it would be desirable to provide an improved method of making consolidated wood composites that do not have undesired soft surfaces.
It is an object of the invention to overcome one or more of the problems described above.
According to the invention a method of making a molded composite article is provided, wherein the method includes the steps of combining a fibrous material with a binder resin and a sizing agent to form a mixture. The method further includes the step of drying the mixture to a moisture content of about 6 percent by weight (wt. %) to about 14 wt. % based on the weight of the fibrous material to form a mat, and thereafter coating a surface of the mat with about 3 grams per square meter (g/m2) to about 50 g/m2 of an aqueous solution. The aqueous solution comprises a wetting agent, a mold release agent, a set retarder, and, optionally, a thermosetting resin. Once coated with the aqueous solution the mat is consolidated under heat and pressure to form the molded composite article.